Images of the interiors of bodies may be acquired using various types of tomographic techniques, which involve recording and measuring radiation from tissues and processing acquired data into images.
One of these tomographic techniques is positron emission tomography (PET), which involves determining spatial distribution of a selected substance throughout the body and facilitates detection of changes in the concentration of that substance over time, thus allowing to determine the metabolic rates in tissue cells.
The selected substance is a radiopharmaceutical administered to the examined object (e.g. a patient) before the PET scan. The radiopharmaceutical, also referred to as an isotopic tracer, is a chemical substance having at least one atom replaced by a radioactive isotope, e.g. 11C, 15O, 13N, 18F, selected so that it undergoes radioactive decay including the emission of a positron (antielectron). The positron is emitted from the atom nucleus and penetrates into the object's tissue, where it is annihilated in reaction with an electron present within the object's body.
The phenomenon of positron and electron annihilation, constituting the principle of PET imaging, consists in converting the masses of both particles into energy emitted as annihilation photons, each having the energy of 511 keV. A single annihilation event usually leads to formation of two photons that diverge in opposite directions at the angle of 180° in accordance with the law of conservation of the momentum within the electron-positron pair's rest frame, with the straight line of photon emission being referred to as the line of response (LOR). The stream of photons generated in the above process is referred to as gamma radiation and each photon is referred to as gamma quantum to highlight the nuclear origin of this radiation. The gamma quanta are capable of penetrating matter, including tissues of living organisms, facilitating their detection at certain distance from object's body. The process of annihilation of the positron-electron pair usually occurs at a distance of several millimeters from the place of the radioactive decay of the isotopic tracer. This distance constitutes a natural limitation of the spatial resolution of PET images to a few millimeters.
A PET scanner comprises detection devices used to detect gamma radiation as well as electronic hardware and software allowing to determine the position of the positron-electron pair annihilation event on the basis of the position and time of detection of a particular pair of the gamma quanta. The radiation detectors are usually arranged in layers forming a ring around object's body and are mainly made of an inorganic scintillation material. A gamma quantum enters the scintillator, which absorbs its energy to re-emit it in the form of light (a stream of photons). The mechanism of gamma quantum energy absorption within the scintillator may be of dual nature, occurring either by means of the Compton's effect or by means of the photoelectric phenomenon, with only the photoelectric phenomenon being taken into account in calculations carried out by current PET scanners. Thus, it is assumed that the number of photons generated in the scintillator material is proportional to the energy of gamma quanta deposited within the scintillator.
When two annihilation gamma quanta are detected by a pair of detectors at a time interval not larger than several nanoseconds, i.e. in coincidence, the position of annihilation position along the line of response may be determined, i.e. along the line connecting the detector centers or the positions within the scintillator strips where the energy of the gamma quanta was deposited. The coordinates of annihilation place are obtained from the difference in times of arrival of two gamma quanta to the detectors located at both ends of the LOR. In the prior art literature, this technique is referred to as the time of flight (TOF) technique and the PET scanners utilizing time measurements are referred to as TOF-PET scanners. This technique requires that the scintillator has a time resolution of a few hundred picoseconds.
Light pulses reaching the scintillator can be converted into electric pulses by means of photomultipliers or photodiodes. Electric signals from the converters carry information on positions and times of the annihilation quanta subject to detection, as well as on the energy deposited by these quanta.
The principal elements of the signal processing system within the radiation detectors are leading edge discriminators and constant fraction discriminators. These elements, combined with time-to-digital converters, facilitate the measurement of time at which the electric signals generated at these detectors exceed a preset reference voltage or a preset signal amplitude fraction, respectively. Said discriminators are built on the basis of standard electronic components and include, among other components, a current source, a preamplifier, a comparator, a shaper, capacitors, resistors, diodes, transistors and transmission lines. If the detector signal is higher than the threshold voltage set at the discriminator, a logical signal is generated at the discriminator output, carrying information on the time at which the gamma quantum was recorded. The charge is measured by means of analog-to-digital converters.
Temporal resolutions of leading edge and constant fraction discriminators are limited by the dependence of the discriminator response on the shape of signals and, in case of leading edge discriminators, also on the amplitude of input signals. Due to the so-called time walk effect, time determined using leading edge discriminators changes along with the signal amplitude. The effect may be adjusted to a certain degree if the signal charge or amplitude is measured simultaneously. In case of constant fraction discriminators, the time at which the signal exceeds the preset amplitude fraction is generally not dependent on the amplitude, but it may change depending on the shape of the signal (i.e on the temporal distribution of photons). The larger the scintillator, the larger the variations in signal shapes and amplitudes.
Logical signals generated at discriminators are processed by means of sequences of logical operations within a triggering system. These operations result in a logical signal providing information on whether the recorded event should be subjected to further electronic processing. The sequences of logical operations are selected depending on the types of detectors, configuration of modules and the frequencies of recorded events; the main objective of these operations is to discard signals that are not useful for image reconstruction and thus to minimize acquisition dead times as well as times required to process the data and reconstruct the images.
The PCT applications WO2011/008119 and WO2011/008118 describe various aspects of PET scanners that may be of relevance for understanding this description, in particular, a method for determining the position of ionization on the basis of the distribution of times or amplitudes of signals measured at different positions along the scintillator. These documents describe solutions that are based on the measurements of the times of flight required for light pulses to reach detector edges.
A US patent application US2009303096 discloses systems and methods for analog to digital conversion. For example, an analog to digital converter is disclosed that includes an analog input that is provided to a comparator bank. The comparator bank receives a reference indicator, and is operable to provide a current output based at least in part on a comparison of the analog input with a reference threshold corresponding to the reference indicator. The analog to digital converter further includes a range selection filter that is operable to receive the current output and to generate the reference indicator based at least in part on a prior output of the comparator bank.
It would be expedient to develop a method and a device for examination of analog signal parameters that would allow to achieve high accuracy of the measurement of the times of arrival of electric pulses, preferably the accuracy of a several dozens of picoseconds, regardless of the shape and the amplitude of these signals, while being characterized by a markedly lower power consumption compared to discriminators of the state of the art and combining the roles of an analog discriminator and a digital transducer. In addition, the discriminator function would combine both leading edge and constant fraction discriminator functions.